The annular cavity that is formed downstream of the impeller of a centrifugal compressor in a turbomachine must be ventilated to take away the heat energy provided by the impeller. This ventilation is usually carried out by taking air from the outlet of the compressor, at the junction between the inlet of an annular diffuser that supplies a combustion chamber.
This diffuser comprises a downstream annular end-piece with a substantially L-shaped axial section whose substantially radial upstream portion extends, with a slight axial clearance, along the downstream face of the impeller, and whose downstream portion is substantially cylindrical. The air taken from the outlet of the compressor flows from outside to inside in the radial annular space formed between the impeller and the radial portion of the end-piece of the diffuser and then travels into the downstream cavity of the impeller.
In operation, the air in the cavity is subjected to considerable centrifugal forces and is driven by the rotation of the impeller which causes a zone of air recirculation in the cavity and a rise of a portion of this air along the downstream face of the impeller, from inside to outside. This air heats up by viscous friction on the downstream face of the impeller and mixes with the tapped-off air coming out of the compressor, increasing the temperature of this air. By convection, this air again heats the downstream face of the impeller, which may reach and exceed a maximum admissible value, which results in a creep of material.
The particular objective of the invention is to provide a simple, effective and economic solution to this problem of the prior art.